Conventional low density polyethylene (LDPE) has good processability, however, when used in film or extrusion coating applications, increased melt strength is still desired.
U.S. Publication No. 2008/0242809 discloses a process for the preparation of a copolymer of ethylene and a comonomer, and where the polymerization takes place in a tubular reactor, at a peak temperature between 290° C. and 350° C. The comonomer is a di- or higher functional (meth)acrylate, and the co monomer is used in an amount between 0.008 mole percent and 0.200 mole percent, relative to the amount of ethylene copolymer.
International Publication No. WO 2007/110127 discloses an extrusion coating composition comprising an ethylene copolymer. The ethylene copolymer is obtained by a polymerization process in a tubular reactor, at a peak temperature between 300° C. and 350° C., and the comonomer is a bifunctional α,ω-alkadiene.
U.S. Pat. No. 5,539,075 discloses the polymerization of ethylene and at least one monomer, which is copolymerizable with ethylene, and includes a polyunsaturated comonomer having a chain of at least eight carbon atoms and at least two non-conjugated double bonds, of which at least one is terminal. The polymerization takes place at a pressure of about 100-300 MPa, and a temperature of about 80°-300° C., under the action of a radical initiator. The polyunsaturated comonomer preferably is an α,ω-alkadiene having 8-16 carbon atoms, and most preferred 1,9-decadiene. Apart from the polyunsaturated comonomer, the polymerization may also involve another vinyl-unsaturated monomer, preferably containing at least one functional group selected from hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups and ester groups. The ethylene copolymers produced have an increased degree of unsaturation, which can be used for cross-linking the ethylene copolymer or grafting reactive groups.
International Publication No. WO 97/45465 discloses an unsaturated ethylene copolymer, a method for producing it, and its use for producing cross-linked structures. The unsaturated ethylene copolymer comprises a polymer obtained by radical polymerization, through a high-pressure process of ethylene and at least one monomer, which is copolymerizable with ethylene, and includes a diunsaturated comonomer of the formula (I): H2C═CH—O—R—CH═CH2, wherein R═—(CH2)m-O—, —(CH2CH2O)n-, or —CH2-C6H10-CH2-O—, m=2-10, and n=1-5. Preferably, the comonomer of formula (I) is 1,4-butanediol divinyl ether.
Tung, L. H., et al., Preparation of Polystyrene with Long Chain Branches via Free Radical Polymerization, J. Polym. Sci., Polym. Chem. Ed., (1981), 19, 2027-39, discloses the use of small amounts of chain-transferring monomers to copolymerize with styrene, free-radically. Of the comonomers examined, vinylbenzylthiol, yielded polystyrene with a branched structure. The branches are disclosed as occurring mainly in the low molecular weight end of the distribution. Vinylbenzylthiol was also found to be an effective agent for the broadening of the molecular weight distribution.
Tung, L. H., Branching Kinetics in Copolymerization of Styrene with a Chain-Transfer Monomer, J. Polym. Sci., Polym. Chem. Ed., (1981), 19, 3209-3217, discloses the use of polymerization kinetics to compute the theoretical molecular weight and degree of branching for the polymerization with styrene with a chain transfer monomer (for example, vinylbenzylthiol).
Liu, J., et al., Branched Polymer via Free Radical Polymerization of Chain Transfer Monomer: A Theoretical and Experimental Investigation, J. Polym. Sci. Part A: Polym. Chem., (2007), 46, 1449-59, discloses a mathematical model for the free radical polymerization of chain transfer monomers containing both polymerizable vinyl groups and telogen groups. The molecular architecture of the polymer is disclosed as being prognosticated according to the developed model, which was validated experimentally by the homopolymerization of 4-vinyl benzyl thiol (VBT), and its copolymerization with styrene.
Other polymerizations are disclosed in WO 2012/057975. However, as discussed, there remains a need for ethylene-based polymers, such as low density polyethylene (LDPE), with improved melt strength, especially for film and extrusion coating applications. These needs and others have been met by the following invention.